1. Field of the Invention
The present invention relates to an optical head for use in an optical disk apparatus that optically records or reproduces information on or from an information recording medium.
2. Description of Related Art
There have been many reports on the optical head for use in the optical disk apparatus.
FIG. 4 schematically depicts a conventional optical head operable with a magnetooptical disk 4 having magnetooptical effect. The optical head shown in FIG. 4 comprises a laser diode 1, a beam splitter 2, an objective lens 3 for focusing a beam on the magnetooptical disk 4, a polarized-beam splitter 5, two optical detectors 6a and 6b, and a differential amplifier 7 for outputting information signals from the magnetooptical disk 4.
The conventional optical head of the above-described structure operates as follows.
A beam emitted by the laser diode 1 is transmitted through the beam splitter 2 and focused by the objective lens 3 mounted in an objective lens driving device (not shown) down to a spot of approximately one micrometer in diameter on a magnetooptical layer of the magnetooptical disk 4. Reflected light from the magnetooptical disk 4 passes through the objective lens 3, is partially reflected by the beam splitter 2, and is incident on the polarized-beam splitter 5. The laser diode 1 is placed so that the direction of polarization is parallel to the sheet (P polarization). For efficient reproduction of information stored in the magnetooptical disk 4, the beam splitter 2 is designed, for example, to have a transmittance of 70% and a reflectance of 30% for P polarization, and a transmittance of 0% and a reflectance of 100% for S polarization. This configuration allows approximately 70% of the beam emitted by the laser diode 1 to be focused on the magnetooptical disk 4. 70% of the P-polarized beam reflected on the magnetooptical disk 4 is transmitted by the beam splitter 2 back to the laser diode 1, whereas the rest 30% is reflected by the beam splitter 2 and reaches the optical detectors 6a and 6b through the beam splitter 5. The reflected beam from the magnetooptical disk 4 bearing information signals induced by the magnetooptical effect is S-polarized. For obtaining a higher signal-to-noise ratio (S/N), the reflectance of the beam splitter 2 for S polarization is designed to be 100%.
The reflected beam from the magnetooptical disk 4 is incident on the beam splitter 5 having an optical axis rotated about the incident beam axis by 45 degrees. The incident beam is split by the beam splitter 5 into two polarized beam components having respective planes of polarization perpendicular to each other. One polarized beam component passes through the beam splitter 5 and is incident on the optical detector 6a, whereas the other is reflected by the beam splitter 5 and reaches the optical detector 6b. Signals from the two optical detectors 6a and 6b are fed to the differential amplifier 7, and its output, having a high S/N, represents the information stored in the magnetooptical disk 4.
Although the conventional optical head described above gives rise to a sufficient performance for detection of magnetooptical information signals by making use of a differential detection technique employing the beam splitter 5, it has a few drawbacks as described below. That is, the optical path of the detection system is almost perpendicular to the optical path of the focusing system, as shown in FIG. 4, due to the characteristics of the beam splitter 2 which is designed to provide a high optical transfer efficiency for the focusing system, a high beam intensity to the optical detectors 6a and 6b, and a high S/N, and is also designed to well balance these factors.
Further, each of the beam splitter 2 and the polarized-beam splitter 5 comprises two polished glass elements bonded together, with one of the bonded faces coated with appropriate multi-thin-films. Such elements are expensive because it is difficult to mass-produce, making it difficult to manufacture a small-sized optical head at a low cost.